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Optimizing OLED efficacy of 2,7-diconjugated 9,9-dialkylfluorenes by variation of periphery substitution and conjugation length

2007-01-05
RATHNAYAKE, Hemali P.
Çırpan, Ali
DELEN, Zeynep
LAHTI, Paul M.
KARASZ, Frank E.
The photoluminescence (PL) and electroluminescence (EL) of four 2,7-bis(phenylethenyl)fluorenes (OFPVs) and two 2,7-diphenylfluorenes (OFPhs) are compared to evaluate effects of nonconjugating peripheral substitution and conjugation length on their EL emissions. The OFPVs exhibit very similar PL spectra with 460-480 nm emission maxima but show large variation in the organic light-emitting diode (OLED) efficacy: from a material that does not give persistent emission in test OLEDs (9,9-diheptyl substitution on the fluorene ring) to materials with luminance efficiencies of 0.5 cd A(-1) and greater (9,9-diethyl substitution on the fluorene ring, methoxy and methoxy/heptyloxy substituents on the phenylethenyl rings). The best OFPV in an ITO/PEDOT-PSS/(emitter)/Ca-Al (ITO: indiurn tin oxide; PEDOT: poly(ethylenedioxythiophene); PSS: poly(styrene sulfonate)) OLED configuration has 9,9-diethyl substitution and terminal heptyloxy substitution (maximum luminance of 1500 cd m(-2) at 12 V). Unlike the OFPVs, the neat OFPhs show not only EL at the desired blue output of ca. 400-410 nm emission maxima but also an undesired green emission component at 500-550 nm. Blending the OFPhs with poly(methyl methacrylate) eliminates the long-wavelength component when the emitter load is 10-25 %, but the OFPh luminance efficiencies, turn-on voltages, and maximum luminance tend to be poorer than those of the OFPVs. The deficiencies of the OFPhs appear to be attributable to thermal degradation and oxidative reactivity, although solid-state annealing and a nonoptimal bandgap match to the OLED device configuration may also contribute.